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A better virtual experience

When the Eyephone (no, that isn’t a typo) made its debut in Silicon Valley in 1987, it was meant to be the wave of the future. This early virtual reality (VR) headset was the brainchild of the computer scientist and technology philosopher Jaron Lanier, who popularized the term “virtual reality” and became, with his company VPL Research, one of the field’s pioneers. But despite the Eyephone’s impressive pedigree and futuristic appeal, its flaws soon became apparent. Its headset consisted of a pair of 3-inch-wide liquid-crystal displays, placed near the focal length of a pair of Fresnel lenses and held in place by a headband. Bulky and awkward to wear, it gave users the rough equivalent of 6/60 vision. Worse, it gave them headaches, nausea and dizziness that sometimes persisted for hours after they had left the virtual world behind. And to top it off, a complete Eyephone system set early adopters back almost $50,000 – in late 1980s dollars.

More than 30 years later, and towards the end of a day-long VR symposium held in San Francisco as part of the 2018 Photonics West conference, Lanier reflected on that first, failed dawn of the VR era. “There’s this crazy thing that in the history of technology, sometimes the first people who come into the field can see further,” he mused. Early VR systems garnered some significant successes, Lanier added, noting that his wife’s life was recently saved by a doctor who trained in a surgical simulator. But when it came to the Eyephone and one of its spin-offs, the PowerGlove, he didn’t mince his words. “It was total crap – are you kidding?” he said, laughing. “We sold thousands of PowerGloves and let us say they were not necessarily great.”

Three decades of intermittent innovation and investment have ironed out many of the Eyephone’s flaws. Consumer devices such as the Oculus Rift VR headset are relatively compact and retail for hundreds of dollars, not thousands. Refresh rates on VR displays often exceed 60 Hz, far better than the handful of frames per second typical of the late 1980s and early 1990s. But reports of user discomfort have not gone away, and as companies such as Microsoft, Google and Intel pour money into VR, it is becoming apparent that some of the technology’s limitations have as much to do with the physics of the human eye as they do with device engineering and design.

Real eyes, virtual world

“Hold your finger up in front of you and look at it,” directs Gordon Love, a physicist who heads the computer science department at Durham University in the UK. “When you do that, a number of things have to happen in your eyes. First of all, you have to focus on your finger. Then you have to verge – that is, point your eyes towards it.” In the real world, Love explains, the location at which your eyes focus, or accommodate, is the same as where they point, or verge: your finger, in this case. In a VR display, though, regardless of whether you are examining a virtual object up close, or marvelling at the lush scenery in the background, your eyes are always focused on the display itself, which is typically a few centimetres away (see figure). Vergence and accommodation are therefore decoupled, which is not the normal state of affairs. “You’re effectively forcing the eye to do something that it never has to do in the real world,” Love says.

A comparison between real-world vision (left) and vision in a typical stereoscopic 3D display (right).

The vergence-accommodation conflict, as it’s known, is not the only source of eyestrain and nausea among VR users. However, decades of research on VR and human vision – dating back to the Eyephone era and even before it – show that it is a major contributor. It is also a problem that industry scientists are working on in earnest. At the Photonics West symposium, Douglas Lanman of Oculus Research – the R&D division of the company behind the Rift headset – placed it third on his list of challenges for VR developers, after improving the resolution of displays and widening their field of view. “Vergence-accommodation is a good research topic, because focusing in a headset is way down developers’ list of priorities,” Lanman explains, adding that the conflict needs to be addressed before VR can achieve its full potential.

Computing tricks

Proposed solutions to the vergence-accommodation conflict come in many forms. Of these, the computational approach is the simplest. To understand how it works, try staring at your finger again. In the real world, if you do this, your finger will be in focus, while everything in the background will be blurred. In a typical VR display, though, all objects appear to be in focus regardless of where your eyes are pointing. The makers of 3D films – who, like VR developers, simulate depth of field by providing slightly different views of a scene to each eye – correct for this computationally, by rendering the important elements in each scene sharply while blurring the background. “Look at a still image from the film Avatar,” Love suggests. “You’ll see that the main character is in focus, while other parts are blurred. And 99% of the time, that’s fine – at least for a film.”

For VR, however, the problem is more complex, because users want to explore and interact with virtual environments and objects rather than passively consuming a pre-recorded scene. Some VR developers have tried blurring images selectively, depending on where the user is looking, but Pierre-Yves Laffont, a computer-vision expert at VR start-up Lemnis Technologies, explains that computational tricks either don’t affect how our eyes accommodate, or they come with significant drawbacks, such as making the entire image blurry. “It’s a cosmetic change,” he says. “It does not really solve the problem.”

Optics to the rescue

The alternative is to tackle the vergence-accommodation conflict optically, by creating VR systems that more nearly mimic how light behaves in the real world. One approach, first suggested in the mid-1990s, is to use multiple displays, each at a different focal plane, to show different parts of a virtual scene. Unfortunately, the additional displays add bulk, and because they are stacked in front of each other, contrast is often lost. “In optics, there is no free lunch and there is no Moore’s Law,” warns Jerry Carollo, an optical architect at Google. “In software, you can do anything, but every time you add an optical adjustment, you’re adding weight and fecking the comfort.”

Real life, with extras: Adding maps to real scenes in a user’s field of view is one application of augmented reality. (Courtesy: iStock/Yagi-Studio)

Another approach is to replace the passive, single-focus optical components found in most VR displays with adaptive multifocal optics. This is the idea that drew Love, whose background is in adaptive optics for astronomy applications, into VR. In 2009 his group worked with Martin Banks’ vision-science lab at the University of California, Berkeley to develop a switchable lens that could change its focal length in less than a millisecond. By placing this lens between the user’s eye and a display, and then rapidly cycling the lens through a handful of different focal lengths, they produced a more realistic accommodation experience, with test subjects reporting fewer negative visual effects.

The main disadvantage with this system, Love says, is that it places strict requirements on the user’s head position. “When your head moves around in real life, you start to see around objects,” he explains. “In our display, when you do that all you see is these different focal planes kind of moving apart, which is really horrible – much worse than the thing you’re trying to correct.” In lab experiments, test subjects bite down on a bar to help keep their heads in place; whether that level of control is possible in a standard VR head-mounted display is, Love says, “an open question”.

Some VR limitations have to do with the physics of the human eye rather than device engineering

A more practical option, at least in the near term, might look something like the prototype that Lemnis Technologies showed off at Photonics West in January 2018. This Singapore-based start-up was founded in 2017, and its team has developed software that can determine, in real time, the optimal location for the focus in each frame of a virtual scene. The Lemnis prototype also incorporates an infrared eye-tracker to monitor where the user is looking, and this information is fed to mechanical actuators that adjust the position of one or more lenses inside the headset – thereby giving users a more “real-world” visual experience. The company is working with undisclosed partners to incorporate its technology into a consumer device, and Laffont, its co-founder and CEO, is naturally confident about its future. “I believe that eye-tracking is a major component of the next generation of VR headsets,” he says, adding that many start-ups in the eye-tracking field have recently been acquired by tech giants such as Facebook, Google and Apple.

Wide-open field

Another strategy for resolving the vergence-accommodation conflict eschews adaptive optics in favour of light-field technology. Conceptually, light-field VR displays work by mapping multiple views of virtual objects onto a single display, with two or more light rays emerging from each incremental area on the object and manifesting as pixels on a screen. The technology is based on a system demonstrated in 1908 by the optical physicist (and Nobel laureate) Gabriel Lippmann, who used an array of small lenses to record a scene as it appeared from many different locations, and additional arrays to reconstruct composite images for each eye. The result was a 3D image that accurately incorporates parallax and perspective shifts – an attractive attribute for VR developers.

There is, however, a problem: the more rays you add, the more pixels you need in your display. “You end up building a low-resolution display, and that’s a big trade to make,” Love explains. Nevertheless, several prototype light-field VR systems have been demonstrated, notably by Nvidia Research. There is also a strong feeling in the industry that light-field technology needs to be perfected for both VR and its newer cousin, augmented reality (AR), to have mass-market applications. In AR, users interact with virtual and real objects simultaneously, and the vergence-accommodation conflict is, if anything, even more bothersome than it is in VR. “Imagine you want to put a virtual object in your hand just in front of you,” explains Laffont. “With today’s AR devices, either your hand will appear sharp or the virtual object will appear sharp, but you won’t be able to focus on both of them at the same time.”

At the Photonics West symposium, Ed Tang, co-founder and CEO of Avegant, a VR developer, was bullish about light-field’s capacity to get around such problems – and, eventually, to dominate the crowded field of VR technology. “Light-field, at the end of the day, is required for the experiences users demand,” he declared. “I would even go so far as to be sceptical that VR will become accepted without light-field.” Others are more cautious. Lanman, who was a senior research scientist at light-field specialists Nvidia until 2014, called light-field displays “academically interesting but very far away”. Laffont echoes this view. “I think light-field displays are really exciting and definitely the future,” he says. “The problem is that the current technology does not allow you to achieve a high field of view and a high resolution together with a low computational expense. And if we have to trade the resolution or the field of view, that will prevent certain applications from being realized.”

For now, both light-field-based VR devices and their adaptive-optics counterparts still look more like modern Eyephones than, well, modern iPhones. A case in point is the Avegant Glyph, a light-field-based video headset that entered the market in 2016. One prominent reviewer, Dieter Bohn of tech site The Verge, summed up the Glyph as “ridiculous-looking, kind of uncomfortable, and pretty awesome”. Two years later, that description remains valid for most – perhaps all – VR headsets available to consumers. “VR initially took off much more slowly than expected,” says Laffont. “We are still at the very early stage of its history.” Despite significant progress and a promising future, the field of VR has a way to go before the virtual becomes truly real.

Stephen Hawking’s scientific legacy

The time: spring 1984. The place: a lecture hall at the University of Cambridge.

Students crowd the seats. In front of the blackboard a stage rises a foot above the floor of the hall. On the stage a man in a wheelchair speaks in a murmur to a graduate student. The student has red tinted glasses and is wearing a soft, cream-coloured jacket. The man: murmur, murmur, murmur. The student: “Professor Hawking says, ‘This lecture is about the edge of the universe.’ ” Murmur, murmur, murmur. “The edge of the universe is very far away.” Murmur, murmur. “Nonetheless, we can still picture it as follows…” The student draws a diagram on the board. The lecture continues, Hawking murmuring, and the student translating. The topic is complex, but the flow is clear. Hawking is an exceptional lecturer, and because everything is said twice, once in a murmur and once in translation, there is extra time to think.

The occasion is Stephen Hawking’s seminar on quantum gravity, and he is talking about his latest work. The audience is rapt and Hawking is excited. He is impatient for the student to finish each translation, and while the student is speaking and writing on the board, Hawking fidgets. Confined by a degenerative motor-neurone disease, he can only move his fingers, which he is using to shift his wheelchair side to side – dzzt, dzzt – as the student speaks – dzzt, dzzt. Each time he moves side to side, however, he also moves imperceptibly backwards. Half an hour into the lecture a huge crash breaks the spell. Hawking’s wheelchair topples off the edge of the stage and is lying on its back on the ground. Hawking’s feet dangle in the air and students jump up to right the chair. He is crumpled at an odd angle, shaking violently. Is he dying? No – he’s laughing. Murmur, murmur, murmur. “Professor Hawking says, ‘Fell off the edge of the universe.’ ”

Hawking led a full and complete life, despite his illness, and his scientific work inspired generations of students to study problems of gravity and quantum mechanics. In the weeks since his death on 14 March, many of his colleagues have written of his remarkable life and work. This article will touch upon a few of his primary scientific accomplishments – in particular, his work on classical gravity and singularities, his famous results on black-hole thermodynamics and Hawking radiation, and his efforts to quantize gravity.

Out of the singularity

As a young man, Hawking worked on general relativity, investigating the problem of how black holes form. His mastery of geometrical methods allowed him to prove a set of remarkable theorems about the circumstances under which swirling clouds of matter undergo gravitational collapse, giving rise to gravitational singularities. Working together with Roger Penrose, who had been investigating similar problems, Hawking produced a remarkable paper in 1970 entitled “The singularities of gravitational collapse and cosmology” (Proc. Roy. Soc. Lond. A 314 529).

Stephen Hawking at the Oxford Students Union. (Courtesy: Roger Askew/Oxford Union/REX/Shutterstock)

In this work, he and Penrose proved that “reasonable” space–times would exhibit both singularities in the future (either the final singularity in a closed universe, or singularities present in black holes) and singularities in the past (the Big Bang). Here, “reasonable” means that the space–time does not possess causal irregularities – such as closed time-like curves – and matter obeys a strong energy condition, so that the presence of matter focuses neighbouring geodesics (the shortest lines between points on a curved surface) towards each other.

Taken together, the Penrose–Hawking singularity theorems provide strong evidence for the necessary formation of black holes in our universe. It is a prediction that has been confirmed both by observational evidence of galactic X-ray sources such as Cygnus X-1, and by the recent and spectacular findings from the Laser Interferometer Gravitational-Wave Observatory (LIGO), which has detected the gravitational waves emitted by colliding black holes.

Hawking’s next major work was his most famous. His singularity theorems implied that the area of a black hole’s event horizon increases when more matter and energy is added to the hole, and should never decrease. Consequently, Hawking and colleagues noted that the area of the horizon was analogous to entropy in the second law of thermodynamics: according to which entropy does not decrease, and tends to increase. Building on Hawking’s ideas, Jacob Bekenstein conjectured that the area of the event horizon was in fact equal to the entropy of the black hole, up to a multiplicative constant.

But now there is a problem. Thermodynamic systems with entropy S(E) as a function of energy E must also possess a temperature, defined by 1/k_BT = ∂S/∂E. But what is the temperature of a black hole? After all, it is black and, at least classically, no radiation can escape from it.

Out of the black hole

In 1974 Hawking solved the problem of black-hole temperature in spectacular fashion. By applying methods of quantum field theory on curved space–time to black-hole geometry, Hawking was able to show that black holes behave like black bodies, emitting thermal radiation with a temperature ħκ/2πk_B (where ħ is Planck’s constant divided by 2π and κ is the surface gravity of the hole), corresponding to a black-hole entropy equal to one quarter of the area of the horizon, measured in Planck units. Hawking’s basic conception of the problem was simple and  brilliant.

General relativity is about the reconciliation of different perceptions of the universe. Special relativity reconciles the perceptions of observers moving at different velocities, but all perceiving the speed of light to be the same. General relativity reconciles the perceptions of different observers who choose to assign different coordinate systems to events in a curved space–time. Once quantum mechanics is thrown into the mix, a remarkable feature arises: inertial observers and accelerated observers have an entirely different perception of the vacuum state – the state with no particles. The vacuum, no-particle state for an inertial observer is perceived by an accelerated observer as containing a thermal mix of  particles.

Hawking radiation artist's concept — Lasting legacy: Hawking transformed our understanding of black holes. (Courtesy: Harald Ritsch/Science Photo Library)

In quantum field theory, the vacuum is not empty – it bubbles with virtual particle–antiparticle pairs. An inertial observer sees those virtual pairs come into existence and then go away again before they can be detected. By contrast, an accelerated observer’s particle detector will detect a thermal mixture of particles – the accelerated detector effectively supplies the energy and momentum needed to create real particles out of virtual particles.

Einstein’s equivalence principle states that acceleration supplied by, for example, a rocket, can’t be distinguished from the acceleration supplied by a gravitational field. Consequently, gravitational fields can create particles. In an elegant argument based on quantum field theory in a black-hole space–time, Hawking was able to show that the gravitational field of the black hole creates a thermal mixture of particles emanating from the black hole’s horizon. In the vicinity of the horizon, virtual particle–antiparticle pairs with entangled energies ±E are “promoted” into real particle–antiparticle pairs of outgoing particles with energy E paired with infalling antiparticles with energy –E. Since the energy of the matter falling into the black hole is negative, the black hole’s mass decreases. The black hole radiates.

Hawking’s discovery that black holes were not actually black stunned the community of physicists. His calculations were confirmed by a variety of alternative approaches, which showed that Hawking radiation was a ubiquitous feature of space–times with horizons, including de Sitter space, and space–time as viewed by an accelerated observer.

The result also spurred interest in the theory of quantum entanglement. Hawking radiation consists of an entangled state of matter, in which outgoing particles with energy +E are entangled with infalling particles with energy –E. The entropy of the Hawking radiation can then be identified with the entanglement entropy of the black hole, where the number of bits of entanglement is proportional to the area of the event horizon.

Hawking radiation is regarded by physicists as the one truly reliable result that we actually possess about quantum mechanics and gravity. Many other results have followed it, notably work on supersymmetric fields of quantum gravity, and the anti-de Sitter space/conformal field theory correspondence (AdS/CFT) – a remarkable connection between the behaviour of gravitational fields in a space–time and the quantum fields on the space–time’s boundary – as well as work to derive general relativity from area laws. However, it seems safe to say that Hawking radiation is the one result on quantum mechanics and gravity that is accepted by the entire community of physicists working on the subject.

Many open questions about Hawking radiation remain. Does the radiation contain the information that went into the black hole as it formed, but in some scrambled form? For decades, Hawking said no – there is apparently no way for the information within the hole to get out, at least under the ordinary laws of quantum mechanics on curved space–time. If the underlying laws of quantum mechanics are information-preserving, as suggested, for example, by the AdS/CFT correspondence, then black-hole evaporation is itself an information-preserving process.

Almost certain escape from a black hole

More recently, however, Hawking stated that he believed that information would actually escape from an evaporating black hole, thereby conceding a bet made with John Preskill on the subject. He went on to declare that his statement that information did not escape from a black hole was his “biggest mistake”. In the absence of a universally accepted, self-consistent quantum theory of gravity, or of empirical evidence for the information-preserving nature of black-hole evaporation, however, the question of how or whether information escapes from black holes must remain open.

Quantum cosmology

In the 1980s Hawking went on to do seminal work on quantum cosmology: his work with Jim Hartle and with others on the quantum theory of universes without boundary represents a conceptually compelling method for approaching the perennially difficult problems of quantum mechanics and the history of the universe as a whole. From this approach Hawking and collaborators were able to obtain useful and suggestive results on quantum cosmology, and cosmological inflation.

The no-boundary approach led to what Hawking had previously called his “biggest mistake”, until he decided that declaring that information didn’t escape from a black hole was worse. Upon using the technique to derive a wave function for the universe that was symmetric in time, Hawking declared that a universe that expanded and then contracted would undergo exact time reversal during the contraction phase. Raymond LaFlamme and Don Page were quickly able to show to Hawking that nothing of the sort need occur. The wave function Hawking had obtained was a quantum superposition of two cosmologies, one starting from a state of low or zero entropy and expanding while entropy increased, and the second being the same cosmology as the first but with time t replaced by –t. If the arrow of time within a universe is to be assigned by the inhabitants, however, then the inhabitants of both universes would likely vote to assign the direction of increasing time in the direction of increasing entropy, so that the second cosmology would be experienced by its inhabitants as being the same as the first, rather than as going backwards in time.

Dancing to the music of time

A physics icon: Stephen Hawking in the 1970s. (Courtesy: The Franklin Institute/AIP Emilio Segrè Visual Archives, Physics Today Collection)

After the publication of his book A Brief History of Time in 1988, and the attendant well-deserved fame, Hawking had less time to devote solely to physics, but he continued to come up with novel and provocative ideas for his entire career. His illness forced him to take things slowly, thinking things out with great thoroughness before putting them forth. But that slow approach served him well.

The scene: a dinner in Mazagon, Spain, at a workshop on the physics of time asymmetry, organized by Jonathan Halliwell, Juan Perez-Mercader and Wojciech Zurek. Hawking is sitting with a bunch of students and postdocs, very slowly eating a very large steak that has been cut up into very small pieces. Everyone is discussing the just-ended conference excursion, which was to a flamenco club in Sevilla. People were asking Stephen how he had ended up dancing with the most accomplished flamenco dancer there. Then the conversation moved on to physics, quantum gravity and so on. Hawking was working away at the keys on his voice synthesizer, and, five minutes later, with a twinkle in his eye, informed us, “I choose my wheelchairs for their…danceability.”

See our special collection on the life and work of Stephen Hawking

Volcanic thunder heard for the first time

Volcanic thunder, long thought to be impossible to hear on account of the intense sounds produced by volcanic eruptions, has been recorded for the first time. A team from the United States Geological Survey (USGS) and Alaska Volcano Observatory captured the sound during a series of eruptions last year at Bogoslof volcano. The rumbles could tell us more about this volcano’s inner workings.

Advances in the understanding of volcanic lightening, which forms as particles rub against each other, have provided a powerful new tool for volcano monitoring, helping scientists work out the amount of ash emitted. Now, volcanic thunder can join the monitoring toolkit.

Volcanic ash poses a major hazard to aircraft. If sucked into a jet engine, the ash may weld together and form a thick concrete, quickly causing the engine to fail. Monitoring ash plume development and advising aircraft to steer clear can prevent such a disaster.

Volcanic thunder indicates an eruption is under way and producing a significant amount of ash. For a long time, other noises caused by the erupting volcano have drowned out the thunder, but Matt Haney from the Alaska Volcano Observatory and his colleagues have found a way to separate the thunder from other activity. The study is accepted for publication in Geophysical Research Letters (GRL).

Using maps of volcanic lightening and a suite of microphones stationed on an island nearby, the team was able to work out exactly where the sound was coming from. “This opens up new ways to detect and analyse volcanic ash plumes,” said Heaney. “I expect that volcanic thunder will be observed at other volcanoes in the future.”

Bogoslof volcano — The most active vent is located under the bay in the centre of the island. Image credit: Dave Schneider, Alaska Volcano Observatory & United States Geological Survey

However, this technique is limited by the precise weather and eruption conditions needed to record the thunder. With over 60 explosive events during Bogoslof’s 2016–17 eruption sequence, only a handful offered the best conditions.

“The optimal conditions are when the wind is low, when you are downwind of the volcano, and when the eruption quiets down abruptly,” said Heaney.

Recording volcanic thunder offers scientists a new way of detecting volcanic lightning and, potentially, a means of estimating the size of an ash plume, but many questions remain. The next step is to understand how specific properties of volcanic ash translate into lightning patterns. Armed with this information, volcanologists could better map the hazard posed to aircraft.

SuperKEKB achieves its first collisions

The first electron–positron collisions occurred at an upgrade to one of Japan’s premier particle-physics experiments on 26 April. Following six years of work, the start of the SuperKEKB accelerator heralds a new era of particle physics at the KEK particle-physics lab in Tsukuba.

SuperKEKB is a ¥29bn ($370m) upgrade of the 3 km circumference KEKB collider, which consists of two circular accelerators – one carrying electrons and the other positrons. KEKB shut down in 2011 for construction to start on SuperKEKB, which involved upgrading the beam-pipes to allow SuperKEKB to produce electrons with an energy of 7 GeV, while the positron beam has an energy of 4 GeV.

More B mesons

The current of the electron ring has also been ramped up from 1.2 A to 2.6 A, with the positron ring boosted from 1.6 A to 3.6 A to increase the number of collision events by a factor of 40 over KEKB. The beam “spot” will be just 50 nm high at the collision point. The upgraded accelerator is designed to pump out large numbers of B mesons (around 50 billion pairs) as well as other particles such as D mesons and tau leptons that could shed further light on why the universe contains more matter than antimatter.

The original KEKB facility included a detector called Belle that allowed physicists to study the remnants of the particle collisions. It has also been upgraded, and the renamed Belle II can handle the huge increase in the collision rate and survive the radiation damage caused by the flux. Key to this design is the inner vertex detector, which has four layers of conventional silicon strips as well as two layers of pixel detectors made from a depleted P-channel field effect transistor (DEPFET). This material, it is hoped, should make the detector better at pinpointing where particles decay.

“Belle II is a unique universal spectrometer with full capabilities for detecting charged particles, photons and neutrinos with high efficiency,” Belle II’s Thomas Browder from the University of Hawaii told Physics World. “This will allow unprecedented sensitivity to the full range of new physics in the ‘flavour sector’”.

Further difficulties

Collisions will now run for six months to refine the accelerator and detector. KEK director general Masanori Yamauchi says: “Though there will be further difficulties that we must face before the SuperKEKB accelerator achieves its design luminosity, 40 times higher than KEKB’s world record, KEK will strive towards success in research together with many collaborators from all over the world”.

The vertex detector will be installed in Belle II by January 2019, with the full physics programme starting a month after that.

Wicked membranes mimic ultrastretchable spider silk

Take a centimetre of silk spun by an ecribellate spider, stretch it to metres and it won’t rupture, thanks to reserves of excess fibre packed into droplets around the thread. The strategy is replicated in other biological systems such as macrophage cells, which can stretch to five times their surface area to engulf large microbes without rupturing. Now researchers in France have spun synthetic membranes with similar reserves of excess material stored in a venous network so that it stays flat over small areas while drawing on the reserves of material to allow ultrastretchable extensions when pulled. The researchers also show how the reservoir structure allows them to add to the functionality of these stretchable membranes.

A number of studies have sought to replicate some of the extraordinary mechanical properties of biomaterials. Arnaud Antkowiak and colleagues at Sorbonne Université, École Normale Supérieure and Saint-Gobain electrospun a fabric of poly(vinylidene fluoride-co-hexafluoropropylene) with fibres around 300 nm thick. They then infused it with a wetting liquid to form the network of veins made up of ruffles of membrane that provide the excess material for ultrastretchable behaviour. Testing the membrane stretched in planar, cylindrical and spherical geometries, the membrane’s behaviour was similar to a soap film and did not rupture.

“The peculiar behaviour of our wicked membrane stems from its compound nature,” explain the researchers in their report. “Capillarity-induced folds allow it to undergo ample shape changes while remaining taut, while its solid underlying matrix provides mechanical robustness.”

Ultrastretchable and some

The researchers also show that the mechanism behind the stretchable characteristics can be put to use to adjust the chemical functionality of structures, by comparing the wettability of a bare zircon bead with one covered in either a hydrophilic or hydrophobic membrane. They also fix 100 nm gold wires to the membrane surface and demonstrate an elementary electronic circuit that can power an LED while being stretched by a factor of eight.

The work exploits one of the many fascinating features of spider silk in a synthetic product. Antkowiak and colleagues suggest that the material may find use in stretchable electronics, flexible batteries, smart textiles, biomedical devices, tissue engineering, and soft robotics applications.

Full details are reported in Science 10.1126/science.aaq0677 .

Grain boundaries limit heat flow in diamond

Imperfections such as grain boundaries in a material affect how efficiently it can transport heat at the nanoscale. A team of researchers led by Kenneth Goodson at Stanford University has now developed the first experimental technique to visualize local variations in heat flow near individual grain boundaries in polycrystalline diamond and has observed a nearly two-fold reduction in local thermal conductivity in the immediate vicinity of the disordered boundaries. This finding will be important for developing materials that are more efficient at carrying away excess heat in electronic devices.

Unwanted heat is a major problem in electronics, and the issue gets even worse as devices become smaller. It is in fact a major stumbling block to miniaturizing components further. To address this challenge, researchers have suggested that synthetic polycrystalline diamond could be ideal as a next-generation heat-sink material. However, understanding how grain boundaries affect heat flow in this material, is crucial to determining how good it might be for thermal management applications.

“Until now, researchers have studied the relationship between grain structure and heat transport using indirect techniques that average over the effects of thousands of grain boundaries. While useful, these measurements provide little direct information about the nature of heat flow in the immediate proximity of an individual boundary, explains team member and study lead author Aditya Sood. “Our approach, which is based on an ultrafast optical pump-probe technique called time-domain thermoreflectance (TDTR), allows us to measure thermal transport down to the level of single grains.”

Constructing 2D maps of thermal conductivity

“The technique involves using short pulses of light, each lasting about a trillionth of a second, to heat up the surface of a material,” he explains. “As heat leaves the surface, we use a second set of pulses to probe the rate at which temperature decays on the nanosecond timescale. This rate of heat flow can be quantitatively linked to the local thermal conductivity of the underlying material.”

By focusing the laser light to a small spot and raster scanning the sample using a high-precision stage, the researchers say they can construct 2D maps of thermal conductivity. In these experiments, one of the main challenges was to be able to precisely locate the grain boundaries in the material and to make sure that the thermal transport measurements were made at the same location on the sample. Working closely with electron microscopy expert Mark Goorsky of the University of California Los Angeles (UCLA), the researchers did this using a technique called electron backscatter diffraction (EBSD), which produces mosaic images of the local crystalline orientation of each grain.

“A unique feature of our work is that we use both techniques in a correlative manner, which allows us to construct a direct visual link between the local thermal conductivity and the underlying grain structure,” Sood tells nanotechweb.org. “This correspondence has never been visualized before with such clarity.”

Two-fold reduction in the local thermal conductivity

The researchers observed a nearly two-fold reduction in the local thermal conductivity of their sample – boron-doped polycrystalline diamond. “This dramatic suppression of heat flow occurs because of the strong scattering of thermal energy carriers (phonons) at the disordered grain boundaries,” explains Sood. “Interestingly, we detect this lowering in thermal conductivity up to a few microns away from the grain boundary. We believe that this non-local effect stems partly from the transport of heat by phonons with long mean-free-paths – that is, atomic vibrations that transmit thermal energy over large distances without scattering.”

The result could be important for thermal management of electronics devices using materials such as high-conductivity synthetic polycrystalline diamond. “Although the specific details are certainly material-dependent, we have shown that heterogeneities in the underlying microstructure of a material need to be considered when evaluating its performance as a heat-sinking substrate.”

“I believe that this is going to become increasingly important as device dimensions start to become comparable to the intrinsic defect length scales in polycrystalline substrates.”

Looking ahead

“In addition to grain boundaries, the new correlative microscopy approach might also come in handy for studying the interactions of vibrational heat carriers with other types of crystalline defects,” says Sood. These could include dislocations, precipitates and pores. “An important field that could benefit from this is thermoelectrics, in which multi-scale defects are intentionally incorporated into materials to impede thermal conduction. Understanding the underlying physics here will require accurate experimental measurements of heat flow near individual defects, something that a technique like ours might provide.”

The team is now busy working on extending its measurements from 2D to 3D. “Many technologically important materials grow with an anisotropic microstructure, such that heat flows very differently within the plane as compared to across the thickness,” explains Sood. “Constructing a 3D microscopic picture of heat transport would allow thermal engineers to design better materials for heat management and routing.”

The Stanford-led project was funded by the Defense Advanced Research Projects Agency (DARPA). As well as the collaboration with Mark Goorsky’s group at UCLA, the Stanford researchers also worked closely with Samuel Graham and colleagues at Georgia Tech. The research is detailed in Nano Letters DOI: 10.1021/acs.nanolett.8b00534.

Synaptic nanomodules could play a role in memory

Changes in the strength between synapses thought to be linked to memory might be more “digital” than previously thought, according to new work by researchers at Thomas Jefferson University in the US. The unexpected result suggests that synaptic plasticity might work by the addition of small same-size modular units of proteins, dubbed nanomodules, rather than by the synaptic connections simply becoming bigger – as was observed before.

Synapses are the biological junctions between neurons and they transform a voltage spike (action potential) arriving from a pre-synaptic neuron into a discharge of chemical neurotransmitters that are then detected by a post-synaptic neuron. These are then transformed into new spikes, leading to a succession of pulses that either become larger or smaller. This fundamental property of synaptic behaviour is known as short-term plasticity, which is related to a neural network’s ability to learn

Unexpected structural changes in spines

As the network learns, the connections between neurons become stronger and this leads to long-lasting changes in the size of the membranous protrusions, or spines, that protrude from neuronal dendrites. These spines, which nearly touch at the synapse, comprise a long thin neck and a rounder swollen head.

To better understand the molecular changes at play in neurons during learning, researchers led by Matthew Dalva employed a technique called multicolour stimulated emission depletion microscopy (STED) to study the connections between neurons that strengthen. Thanks to their images, the researchers have discovered hitherto unseen structural changes in the spines as they become bigger during learning.

Two and three colour STED

“Our technique relies on two and three colour STED to image how key synaptic proteins are organized on the nanoscale,” explains Dalva. “We were able to combine this approach with confocal imaging to examine the relationship between the morphology, or shape, of the dendritic spines and how the proteins that regulate spine function are organized.”

As well as this work, which was done on fixed cells, the researchers also conducted two-colour STED combined with confocal imaging to visualize what happens at spines in living neurons. “In these experiments, we induced spine plasticity (that is, we made the spine get bigger) and watched how the nanostructures at the synapses changed.”

Nanomodules begin to jiggle and move

Team member Martin Hruska’s experiments revealed that as spines increase in size, they add modular units of synaptic proteins, dubbed nanomodules. “This result suggests that changes in synaptic strength – changes that scientist think are linked to memory – might be more digital than previously thought,” Dalva tells nanotechweb.org. “The finding, which will need to be confirmed in future work, suggests that synaptic plasticity might work by adding small same-size units of proteins rather than the synaptic connections themselves simply scaling up in a linear fashion.”

And that was not all: “we also found that the nanomodules begin to jiggle and move around the synaptic spine, with pre- and post-synaptic connections always in lock step,” says Hruska.

Many new questions to be answered

The research, which is detailed in Nature Neuroscience doi:10.1038/s41593-018-0138-9, raises many interesting questions, adds Dalva. “In this work, we only looked a few different types of synaptic proteins, for instance. Are others also organized in the same way? How are the nanomodules we observed generated, and how do they stay aligned across the synaptic cleft between neurons? And how do the nanomodules increase in number?

“Could this work help us better understand how synapses work? For example, does each nanomodule operate independently or are they somehow functionally linked? These are all key questions we plan to examine next.

“Finally, our observations could also help us better understand the maladaptive learning that occurs in disorders such as addiction or other neurological diseases, in which abnormally strong or weak connections between neurons are thought to be responsible,” he adds. “It is yet difficult to say how the nanomodules behave in disease states, but our work offers a new way to explore those questions.”

Start-ups on show at ESTRO

The ESTRO 37 meeting in Barcelona last week saw over 100 companies exhibiting their products on the show floor. The 5401 m² exhibition included a corner dedicated to start-up companies. Here, we take a look at the innovation being developed by start-ups Neolys Diagnostics, Medical Precision, PhantomX and NU-RISE.

Radiation therapy just got personal
Lyon-based Neolys Diagnostics is developing decision-making tools that allow radiation oncologists to adapt treatment according to the radiosensitivity of an individual patient. The aim is to reduce treatment side effects while optimizing efficacy.

The toxicity of radiotherapy is dependent upon three factors: the dose delivered to cells, the cell environment (such as the presence of chemotherapeutics) and the radiosensitivity of the patient. “For many years, there have been improvements mostly in the first of these factors, but the individual’s radiosensitivity has been completely forgotten,” explained Gilles Devillers, the company’s president and co-founder. “Still today, 20% of patients receiving radiotherapy have side effects.”

Neolys Diagnostics' Julien Gillet-Daubin and Gilles Devillers — Neolys Diagnostics’ CEO Julien Gillet-Daubin (left) and president Gilles Devillers at ESTRO 37.

To address this shortfall, Neolys Diagnostics currently offers two test systems. The first, RadioDTect, involves a simple blood test that gives results in 4-5 hours. Designed to be performed for every patient, at the cancer centre, the test identifies whether a patient is at risk of experiencing severe side effects or not. “This test reveals whether a patient is radiosensitive, but it doesn’t show to what extent,” Devillers explained.

To create a detailed map of their level of risk, some patients will require the next step – RadioProfile. This high-performance test involves a small skin biopsy and is performed in Expert Labs partnering with Neolys Diagnostics. Within 2-3 weeks, the test will predict exactly which side-effects a patient will have and at what level.

The tests work by examining DNA repair following radiation-induced double-strand breaks. The number of unrepaired versus repaired breaks (among other markers), identified using immunofluorescence analysis, depend upon the function of the ATM protein in the patient and are a reliable indicator of an individual’s radiosensitivity. This allows estimation of the radiation dose that the patient can tolerate without adverse side effects.

Devillers notes that both tests are CE marked and that seven additional clinical trials are ongoing. In the next phase of development, Neolys Diagnostics is developing a decision support system that helps clinicians choose the most suitable treatment for their patient, providing justification for a particular modality, such as proton therapy, for example, or hypofractionation. “We are going to bring the missing part of the radiotherapy toxicity assessment,” said Devillers.

Reference points without the pain

While radiotherapy delivery techniques have undergone enormous advancements in recent years, the reference points used to guide treatment are still created using tattoos. These are painful for the patient and are commonly injected at depths of 2-3 mm, meaning that they remain forever. This permanence can be particularly distressing for patients with several tattoos in prominent locations, such as breast cancer patients, for example.

Dutch start-up Medical Precision has come up with an alternative. Its MPV-16 system, based on technology developed for permanent make-up, uses a tiny needle to inject pigment into the upper dermis. This painless procedure, where the patient just feels the needle vibration, still creates a highly visible skin mark. Importantly, the mark is not permanent and will disappear six months to a year later – or it can be removed immediately using a laser.

The CE-approved device comes with three colour-coded heads, each offering a different injection depth. The smallest-depth needle, for thin skin, penetrates just 0.2 mm into tissue. For darker or thicker skin, a needle that penetrates 0.4 mm can be used. If a permanent mark is required, a 0.7 mm option is available. Even at this depth, the process is still pain free.

As well as being better for the patient, the shallow depth of the mark also confers greater accuracy for patient positioning. “If a tattoo is too deep, it spreads out under the skin,” explained Annelies Maas, the company’s CEO. “Using a depth of 0.7 mm still creates a small dot that doesn’t grow. The mark stays in place, but can also be removed using a laser.”

Earlier this month, Medical Precision partnered with CIVCO Radiotherapy, who will be the distributors for this innovative marking system.

Copying patients into phantoms

PhantomX, a spin-off from Charité – Universitätsmedizin Berlin, has created a novel 3D printing method that can “copy patients into phantoms”. The idea is to create a phantom with exactly the same radiation attenuation as a patient, enabling realistic simulation of their radiation exposure in CT and radiation therapy.

The phantoms are created using a two-step process. First, CT images of the patient’s anatomy are printed in radiopaque ink on paper, with the CT grayscale used to encode radiation attenuation levels. Then the paper images are stacked into a 3D structure and cut to shape. Following this process, a CT scan of the phantom will be identical to that of the patient.

Paul Jahnke and Tetje Dietrich from PhantomX — Paul Jahnke (left) and Tetje Dietrich from PhantomX with a head phantom and a hand phantom.

One key application is patient-specific radiotherapy quality assurance (QA). Here, a treatment plan is delivered to the phantom on the linac and the resulting dose is measured. The phantoms can also be used to generate realistic CT and X-ray images, to investigate dose and imaging parameters. Finally, they provide a realistic simulation environment for training of medical staff.

According to project leader Paul Jahnke, the phantoms could be used for all patient treatments. They are, however, particularly relevant for treatments of younger patients, secondary tumours, head-and-neck tumours near organs-at-risk, or patients with comorbidities.

Optical probes enable in vivo dosimetry

NU-RISE of Portugal is developing an in vivo dosimetry system based on fibre-optic probes that are placed inside the patient’s body. The probes monitor the level of radiation that the patient is receiving in real time, helping doctors better target the cancer and protecting patients from unnecessary irradiation. The company’s vision is to provide valuable data in real time for clinical decision support, enabling safer and accurate radiation treatments.

“We are working to solve one of radiotherapy’s problems – to know what happens in the patient in real time,” explained Luís Moutinho, NU-RISE’s founder and CEO. “Our system can precisely measure the amount of radiation delivered to a tiny volume, usually where organs-at-risk are, and compare it with the treatment plan.”

NU-RISE's in vivo dosimetry system — NU-RISE’s *in vivo* dosimetry system.

The system uses optical probes, each of which contains at its end a 2 mm section of a material that converts radiation to light. The probes are less than 1 mm in diameter, making them compatible with standard catheters. This makes NU-RISE’s dosimeter a great solution for real-time in vivo dosimetry in brachytherapy, where currently there are no options available. NU-RISE has also developed a novel method to process the resulting optical signal with minimal noise. According to Moutinho, this allows the probes to cost 10-20 times less than other in vivo dosimetry probes.

This lower pricing level enables the probes to be disposable. As such, each probe is provided pre-calibrated, sterilized and bar-coded, such that each fibre can be easily identified via scanning. The system enables up to six probes to be placed and used simultaneously for each treatment.

The company is currently working to CE Mark the device, but the dosimetry system is available for use by NU-RISE’s clinical partnerships and is already being employed in patients. NU-RISE is also looking for new collaborations and applications of its technology.

Physicists explain why clothes do not fall apart

A new twist in our understanding of how fibres hold together in yarn and other spun materials has been revealed by three physicists in the UK. Using statistical physics to study a problem previously contemplated in 1638 by none other than Galileo, the trio has modelled how yarn makes a transition from being a weak material that can be easily pulled apart, to a much stronger material that only breaks when its fibres snap.

Few of us stop to think whether the spun and woven fibres that make our clothes will separate from one another, sparing no blushes as they fall in a heap on the floor. Although it is known that multiple frictional contact points between fibres hold clothes, ropes and yarn together, the mechanics behind why these fibre structures do not slip apart remains unclear.

Tension transfer

Patrick Warren at Unilever, Robin Ball at the University of Warwick and Raymond Goldstein of the University of Cambridge set out to answer this question by focussing on staple yarns. These are made of spun fibres, which are individually about 2-3 cm long and comprise wool, cotton or linen. The hypothesis of their study is that a yarn with few contact points between its fibres will fail by the fibres slipping past each other. However, they postulate, twisting the fibres together provides sufficient contact points so that normal forces acting between pairs of fibres allow tension to be transferred between them. This, they predict, should transform a weak yarn into a much stronger (up to 100-fold) material that can only fail by the fibres snapping.

To provide evidence for their hypothesis, the team translated the textile mechanics problem into an abstract linear programming problem. This involves treating the yarn as a collection of randomly overlapping near-parallel fibres and solving a system of linear inequalities representing the set of tensions in each of the fibre elements.

As a quantity representing the average tension transfer between fibres grew, the researchers witnessed a transition appear in their model, where the inequalities went from being unsatisfied to satisfied. In terms of the yarn, this represents the hypothesized transition from the fibres being free to slip, to a state in which the fibres are collectively locked together by friction, “The mechanical integrity reflects a genuine, and generic, phase transition in terms of a collective friction locking mechanism in fibre assemblies,” summarizes Goldstein.

Industry experience

Further statistical analysis revealed that this transition is smooth and depends on a combination of friction coefficient and turning angle per contact – where the turning angle is the number of twists in a fibre and a contact is a location where fibres are pinned together by friction. This provides a theoretical footing for what industry experience had shown – that yarn lubrication (which reduces the friction coefficient) and spinning (which increases the turning angle) are key “make or break” factors in the manufacturing process.

Physicists explain why clothes do not fall apart

Although various approaches have been taken in the past that attempt to work-out how fibrous textile materials become so strong, Goldstein believes their statistical physics abstract model is “a novel application of linear programming, which could be transferrable to other situations involving static friction”. An important application could be the study of how stress is transmitted in granular materials – which could provide insight into everyday phenomena such as the behaviour of sand in piles and grain in silos.

Ning Pan of the University of California, Davis describes the research as a “fresh characterization” that applies new tools like percolation theory and linear programming to a long-standing problem. However, he also urges caution in how the work is interpreted “The abstract model is highly simplified and ‘unorthodox’, so it would be hard for future research to build on.”

The research is described in Physical Review Letters.

Penguin physics, talking about diversity issues, millionth passenger tours exoplanets

In case you missed it, Wednesday was World Penguin Day and here at Physics World we love a good story about the antipodean birds. In “Penguin physics” the University of Bristol polymer physicist Peter Barham describes how he got involved in designing flipper bands that are used to study the behaviour of penguins. He gives a taste of his work in the above video.

Recently, we wrote about how a colony of nesting king penguins resembles a liquid that has been cooled suddenly to form a glass-like structure. Indeed, the physicists who did that research have an upcoming paper about phase transitions in emperor penguin colonies – so stay tuned for more.

LGBT+ survey

Our colleague Angela Townsend, who co-ordinates the diversity programme at the Institute of Physics, e-mailed us this week with details of a fascinating interview with David Smith from the University of York in Chemical and Engineering News. Smith’s a chemist who’s into supramolecular and nanoscale chemistry, which isn’t physics, but is close enough not to give anyone on this website a fright. Townsend and Smith have worked closely on the recently formed LGBT+ Physical Sciences Network, which is currently carrying out a survey into the “working, teaching and studying climate” for LGBT+ physicists, chemists and astronomers in the UK (closing date: 30 April).

In the interview, Smith describes his own story of how he came out as a gay chemist and how, when he began his research career in the early 1990s, he knew no other gay chemists at all. And despite Smith not finding it easy talking about diversity issues, he thinks it’s important to do so. “PhD students and postdocs in particular imagine you have to tread this cookie-cutter path to get to where you’re going,” he says. “And I think a lot of people hiring academics think the same. Anything you can do to break down that view is very powerful.”

Since it was published in September 2017, the above video from the UK’s University of Exeter and We the Curious in Bristol has attracted more than one million views. Described as a “Stunning virtual tour of exoplanets“, the virtual-reality video takes you to six different exoplanets and also contains a good dose of astrophysics.

We the Curious is a hands-on science exhibition centre. Its creative director Anna Starkey was featured in our “Once a physicist” careers column, which charts the careers of physicists working in far-flung fields.